Miniaturized atomic clocks characterized by a small size and a drastically reduced power consumption compared to standard atomic clocks exhibit an increasing interest mainly for applications in portable devices. The unprecedented frequency stability of atomic clocks is achieved by a suitable interrogation of optically excited atoms which takes place in the so-called vapor cell, the heart of an atomic clock. The vapor cell consists of a sealed cavity which contains small amounts of an alkali metal, preferably rubidium or cesium, a buffer gas and/or an anti-relaxation coating. MEMS (Microelectromechanical systems) technology allows the fabrication of miniaturized vapor cells having a volume in the range of a few cubic millimeters. The fabrication of vapor cells typically consists in etching through holes into a substrate, as a silicon wafer, bonding a glass wafer onto one side of the silicon wafer, filling the cavity with an alkali metal, and closing the cavity by bonding a second glass wafer on the other side of the silicon wafer. Such a method is disclosed for example in the patent publication US 2005/0007118. The difficulties encountered during the fabrication of vapor cells are related to the volatile character of alkali metals and to the reactivity of alkali metals with oxygen. As a result, all handling of alkali metals has to be done either under high vacuum conditions or in an anaerobic atmosphere, a fact that complicates the fabrication of alkali metal vapor cells.
Several fabrication approaches have been reported which can be categorized in four different groups:                a) cell filling using commercially available alkali metal dispensers;        b) cell filling using the chemical reaction of barium azide and rubidium or cesium chloride producing metallic rubidium or cesium, barium chloride, and elementary nitrogen. The chemical reaction can take place in situ or ex situ;        c) cell filling using alkali metal azide deposited by vacuum thermal evaporation followed by thermal- or UV-decomposition to produce pure alkali metal and elementary nitrogen. The decomposition can take place in situ or ex situ;        d) electrolytic decomposition of alkali metal enriched glass.        
From the four cell filling approaches a) to d), methods b) to d) can be potentially scaled-up to wafer-level filling. Li-Anne Liew et al., in the publication “Microfabricated alkali atom vapor cells”, Appl. Phys. Lett., Vol. 84, No. 14, 5 Apr. 2004, describe a cell filling method using an aqueous solution of dissolved BaN6 and CsCl and further in situ chemical reaction to form BaCl, metallic Cs, and nitrogen (method b). A disadvantage of such method is that unreacted Ba tends to form different forms of nitride with the released nitrogen, causing pressure fluctuation inside the cell which affects the stability of the atomic clock, as disclosed by S. Knappe et al. in the publication “Atomic vapor cells for chipscale atomic clocks with improved long-term frequency stability”, Opt. Lett. 30, 2351-3.
Li-Anne Liew et al., in the publication “Wafer-level filling of microfacricated atomic vapor cells based on the thin-film deposition and photolysis of cesium azide”, Appl. Phys. Lett. 90, 114106 (2007), describe a method for thin-film deposition of cesium azide (CsN3) by vacuum thermal evaporation using custom built evaporation equipment and further in situ decomposition (method c). However, this method is hazardous since, for thermal evaporation, the azide has to be heated above its melting point, favoring uncontrolled decomposition and explosion.
For batch fabrication, the cell filling by manually placing solid crystals of alkali metal azide into cavities is cumbersome and an accurate control of the deposited amount of azide is impossible. The method described in the publication of Li-Anne Liew et al. cited above solves some of these challenges but suffers from cost and danger related disadvantages.
Hence methods have to be found that allow an easy, cost effective and precise cell filling.